Transcoding Services Invocation in the Session Initiation Protocol (SIP) Using Third Party Call Control (3pcc)

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Copyright (C) The Internet Society (2005).

Abstract

This document describes how to invoke transcoding services using Session Initiation Protocol (SIP) and third party call control. This way of invocation meets the requirements for SIP regarding transcoding services invocation to support deaf, hard of hearing and speech-impaired individuals.

The framework for transcoding with SIP [4] describes how two SIP [1] UAs (User Agents) can discover incompatibilities that prevent them from establishing a session (e.g., lack of support for a common codec or common media type). When such incompatibilities are found, the UAs need to invoke transcoding services to successfully establish the session. 3pcc (third party call control) [2] is one way to perform such invocation.

In the 3pcc model for transcoding invocation, a transcoding server that provides a particular transcoding service (e.g., speech-to-text) is identified by a URI. A UA that wishes to invoke that service sends an INVITE request to that URI establishing a number of media streams. The way the transcoder manipulates and manages the contents of those media streams (e.g., the text received over the text stream is transformed into speech and sent over the audio stream) is service specific.

All the call flows in this document use SDP. The same call flows could be used with another session description protocol that provides similar session description capabilities.

Given two UAs (A and B) and a transcoding server (T), the invocation of a transcoding service consists of establishing two sessions; A-T and T-B. How these sessions are established depends on which party, the caller (A) or the callee (B), invokes the transcoding services.Section 3.2 deals with callee invocation and Section 3.3 deals with caller invocation.

In all our 3pcc flows we have followed the general principle that a 200 (OK) response from the transcoding service has to be received before contacting the callee. This tries to ensure that the transcoding service will be available when the callee accepts the session.

Still, the transcoding service does not know the exact type of transcoding it will be performing until the callee accepts the session. So, there is always the chance of failing to provide transcoding services after the callee has accepted the session. A system with more stringent requirements could use preconditions to avoid this situation. When preconditions are used, the callee is not alerted until everything is ready for the session.

SDP A: A session description generated by A. It contains, among other things, the transport address/es (IP address and port number) where A wants to receive media for each particular stream.

SDP B: A session description generated by B. It contains, among other things, the transport address/es where B wants to receive media for each particular stream.

SDP A+B: A session description that contains, among other things, the transport address/es where A wants to receive media and the transport address/es where B wants to receive media.

SDP TA: A session description generated by T and intended for A. It contains, among other things, the transport address/es where T wants to receive media from A.

SDP TB: A session description generated by T and intended for B. It contains, among other things, the transport address/es where T wants to receive media from B.

SDP TA+TB: A session description generated by T that contains, among other things, the transport address/es where T wants to receive media from A and the transport address/es where T wants to receive media from B.

In this scenario, B receives an INVITE from A, and B decides to introduce T in the session. Figure 1 shows the call flow for this scenario.

In Figure 1, A can both hear and speak, and B is a deaf user with a speech impairment. A proposes to establish a session that consists of an audio stream (1). B wants to send and receive only text, so it invokes a transcoding service T that will perform both speech-to-text and text-to-speech conversions (2). The session descriptions of Figure 1 are partially shown below.

When either A or B decides to terminate the session, it sends a BYE indicating that the session is over.

If the first INVITE (1) received by B is empty (no session description), the call flow is slightly different. Figure 2 shows the messages involved.

B may have different reasons for invoking T before knowing A's session description. B may want to hide its lack of native capabilities, and therefore wants to return a session description with all the codecs that B supports, plus all the codecs that T supports. Or T may provide recording services (besides transcoding), and B wants T to record the conversation, regardless of whether transcoding is needed.

This scenario (Figure 2) is a bit more complex than the previous one. In INVITE (2), B still does not have SDP A, so it cannot provide T with that information. When B finally receives SDP A in (6), it has to send it to T. B sends an empty INVITE to T (7) and gets a 200 OK with SDP TA+TB (8). In general, this SDP TA+TB can be different than the one sent in (3). That is why B needs to send the updated SDP TA to A in (9). A then sends a possibly updated SDP A (10) and B sends it to T in (12). On the other hand, if T happens to return the same SDP TA+TB in (8) as in (3), B can skip messages (9), (10), and (11). So, implementors of transcoding services are encouraged to return the same session description in (8) as in (3) in this type of scenario. The session descriptions of this flow are shown below:

In this scenario, A wishes to establish a session with B using a transcoding service. A uses 3pcc to set up the session between T and B. The call flow we provide here is slightly different than the ones in [2]. In [2], the controller establishes a session between two user agents, which are the ones deciding the characteristics of the streams. Here, A wants to establish a session between T and B, but A wants to decide how many and which types of streams are established. That is why A sends its session description in the first INVITE (1) to T, as opposed to the media-less initial INVITE recommended by [2]. Figure 3 shows the call flow for this scenario.

We do not include the session descriptions of this flow, since they are very similar to those in Figure 2. In this flow, if T returns the same SDP TA+TB in (8) as in (2), messages (9), (10), and (11) can be skipped.

Sometimes, as pointed out in the requirements for SIP in support of deaf, hard of hearing, and speech-impaired individuals [5], a user wants to receive both the original stream (e.g., audio) and the transcoded stream (e.g., the output of the speech-to-text conversion). There are various possible solutions for this problem. One solution consists of using the SDP group attribute with Flow Identification (FID) semantics [3]. FID allows requesting that a stream is sent to two different transport addresses in parallel, as shown below:

The problem with this solution is that the majority of the SIP user agents do not support FID. Moreover, only a small fraction of the few UAs that support FID, also support sending simultaneous copies of the same media stream at the same time. In addition, FID forces both copies of the stream to use the same codec.

Camarillo, et al. Informational [Page 9]

RFC 4117 3pcc Transcoding in SIP June 2005

Therefore, we recommend that T (instead of a user agent) replicates the media stream. The transcoder T receiving the following session description performs speech-to-text and text-to-speech conversions between the first audio stream and the text stream. In addition, T copies the first audio stream to the second audio stream and sends it to A.

Transcoding services sometimes consist of human relays (e.g., a person performing speech-to-text and text-to-speech conversions for a session). If the same person is involved in both conversions (i.e., from A to B and from B to A), he or she has access to all of the conversation. In order to provide some degree of privacy, sometimes two different persons are allocated to do the job (i.e., one person handles A->B and the other B->A). This type of disposition is also useful for automated transcoding services, where one machine converts text to synthetic speech (text-to-speech) and another performs voice recognition (speech-to-text).

The scenario described above involves four different sessions: A-T1, T1-B, B-T2 and T2-A. Figure 4 shows the call flow where A invokes T1 and T2.

Note this example uses unidirectional media streams (i.e., sendonly or recvonly) to clearly identify which transcoder handles media in which direction. Nevertheless, nothing precludes the use of bidirectional streams in this scenario. They could be used, for example, by a human relay to ask for clarifications (e.g., I did not get that, could you repeat, please?) to the party he or she is receiving media from.

Note that B, the user agent server, needs to support two media streams: sendonly and recvonly. At present, some user agents, although they support a single sendrecv media stream, do not support a different media line per direction. Implementers are encouraged to build support for this feature.

In a distributed environment, a complex transcoding service (e.g., English text to Spanish speech) is often provided by several servers. For example, one server performs English text to Spanish text translation, and its output is fed into a server that performs text- to-speech conversion. The flow in Figure 5 shows how A invokes T1 and T2.

RFC 3725 [2] discusses security considerations which relate to the use of third party call control in SIP. These considerations apply to this document, since it describes how to use third party call control to invoke transcoding service.

In particular, RFC 3725 states that end-to-end media security is based on the exchange of keying material within SDP and depends on the controller behaving properly. That is, the controller should not try to disable the security mechanisms offered by the other parties. As a result, it is trivially possible for the controller to insert itself as an intermediary on the media exchange, if it should so desire.

In this document, the controller is the UA invoking the transcoder, and there is a media session established using third party call control between the remote UA and the transcoder. Consequently, the attack described in RFC 3725 does not constitute a threat because the controller is the UA invoking the transcoding service and it has access to the media anyway by definition. So, it seems unlikely that a UA would attempt to launch an attack against its own session by disabling security between the transcoder and the remote UA.

Regarding end-to-end media security from the UAs' point of view, the transcoder needs access to the media in order to perform its function. So, by definition, the transcoder behaves as a man in the middle. UAs that do not want a particular transcoder to have access to all the media exchanged between them can use a different transcoder for each direction. In addition, UAs can use different transcoders for different media types.

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